Early Greek Science: Thales to Plato Michael Fowler, Uva Physics, 8/23/08
Total Page：16
File Type：pdf, Size：1020Kb
previous index next Early Greek Science: Thales to Plato Michael Fowler, UVa Physics, 8/23/08 The Milesians The first recorded important contributions to Greek science are from the city of Miletus, near the coast of what is now Turkey, beginning with Thales in about 585 B.C., followed by Anaximander about 555 B.C., then Anaximenes in 535 B.C. We shall argue below that these Milesians were the first to do real science, immediately recognizable as such to a modern scientist, as opposed to developing new technologies. The crucial contribution of Thales to scientific thought was the discovery of nature. By this, we mean the idea that the natural phenomena we see around us are explicable in terms of matter interacting by natural laws, and are not the results of arbitrary acts by gods. An example is Thales’ theory of earthquakes, which was that the (presumed flat) earth is actually floating on a vast ocean, and disturbances in that ocean occasionally cause the earth to shake or even crack, just as they would a large boat. (Recall the Greeks were a seafaring nation.) The common Greek belief at the time was that the earthquakes were caused by the anger of Poseidon, god of the sea. Lightning was similarly the anger of Zeus. Later, Anaximander suggested lightning was caused by clouds being split up by the wind, which in fact is not far from the truth. The main point here is that the gods are just not mentioned in analyzing these phenomena. The Milesians’ view is that nature is a dynamic entity evolving in accordance with some admittedly not fully understood laws, but not being micromanaged by a bunch of gods using it to vent their anger or whatever on hapless humanity. 2 An essential part of the Milesians’ success in developing a picture of nature was that they engaged in open, rational, critical debate about each others ideas. It was tacitly assumed that all the theories and explanations were directly competitive with one another, and all should be open to public scrutiny, so that they could be debated and judged. This is still the way scientists work. Each contribution, even that of an Einstein, depends heavily on what has gone before. The theories of the Milesians fall into two groups: (1) theories regarding particular phenomena or problems, of the type discussed above, (2) speculations about the nature of the universe, and human life. Concerning the universe, Anaximander suggested that the earth was a cylinder, and the sun, moon and stars were located on concentric rotating cylinders: the first recorded attempt at a mechanical model. He further postulated that the stars themselves were rings of fire. Again, a very bold conjecture—all heavenly bodies had previously been regarded as living gods. He also considered the problem of the origin of life, which is of course more difficult to explain if you don’t believe in gods! He suggested that the lower forms of life might be generated by the action of sunlight on moist earth. He also realized that a human baby is not self-sufficient for quite a long time, so postulated that the first humans were born from a certain type of fish. All three of these Milesians struggled with the puzzle of the origin of the universe, what was here at the beginning, and what things are made of. Thales suggested that in the beginning there was only water, so somehow everything was made of it. Anaximander supposed that initially there was a boundless chaos, and the universe grew from this as from a seed. Anaximenes had a more sophisticated approach, to modern eyes. His suggestion was that originally there was only air (really meaning a gas) and the liquids and solids we see around us were formed by condensation. Notice that this means a simple initial state develops into our world using physical processes which were already familiar. Of course this leaves a lot to explain, but it’s quite similar to the modern view. Early Geometry One of the most important contributions of the Greeks was their development of geometry, culminating in Euclid’s Elements, a giant textbook containing all the known geometric theorems at that time (about 300 BC), presented in an elegant logical fashion. Notice first that the word “geometry” is made up of “geo”, meaning the earth, and “metry” meaning measurement of, in Greek. (The same literal translations from the Greek give geography as picturing the earth (as in graphic) and geology as knowledge about the earth. Of course, the precise meanings of all these words have changed somewhat since they were first introduced.) 3 The first account we have of the beginnings of geometry is from the Greek historian Herodotus, writing (in 440 B.C. or so) about the Egyptian king Sesotris (1300 B.C.): “This king moreover (so they said) divided the country among all the Egyptians by giving each an equal square parcel of land, and made this the source of his revenue, appointing the payment of a yearly tax. And any man who was robbed by the river of a part of his land would come to Sesotris and declare what had befallen him; then the king would send men to look into it and measure the space by which the land was diminished, so that thereafter it should pay the appointed tax in proportion to the loss. From this, to my thinking, the Greeks learnt the art of measuring land...” On the other hand Aristotle, writing a century later, had a more academic, and perhaps less plausible, theory of the rise of geometry: “..the sciences which do not aim at giving pleasure or at the necessities of life were discovered, and first in the places where men first began to have leisure. That is why the mathematical arts were founded in Egypt, for there the priestly class was allowed to be at leisure.” However, as Thomas Heath points out in A History of Greek Mathematics, (page 122) one might imagine that if this (that is, Aristotle’s theory) were true, Egyptian geometry “would have advanced beyond the purely practical stage to something more like a theory or science of geometry. But the documents which have survived do not give any grounds for this supposition; the art of geometry in the hands of the priests never seems to have advanced beyond mere routine. The most important available source of information about Egyptian mathematics is the Papyrus Rhind written probably about 1700 BC, but copied from an original of the time of King Amenemhat III (Twelfth Dynasty), say 2200 BC.” Heath goes on to give details of what appears in this document: areas of rectangles, trapezia and triangles, areas of circles given as (8d/9)2, where d is the diameter, corresponding to pi equal to 3.16 or so, about 1% off. There are approximate volume measures for hemispherical containers, and volumes for pyramids. Another important Egyptian source is the Moscow Papyrus, which includes the very practical problem of calculating the volume of a pyramid! (Actually with a flat top: look at the figure, from Wikipedia.) A brief overview of the early history of geometry, up to Euclid, has been written by the Greek author Proclus. He asserts that geometry was first brought to Greece by Thales, after he spent some years in Egypt. 4 The Pythagoreans: a Cult with a Theorem, and an Irrational Discovery Pythagoras was born about 570 B.C. on the island of Samos (on the map above), less than a hundred miles from Miletus, and was thus a contemporary of Anaximenes. However, the island of Samos was ruled by a tyrant named Polycrates, and to escape an unpleasant regime, Pythagoras moved to Croton, a Greek town in southern Italy (at 39 05N, 17 7 30E), about 530 B.C. Pythagoras founded what we would nowadays call a cult, a religious group with strict rules about behavior, including diet (no beans), and a belief in the immortality of the soul and reincarnation in different creatures. This of course contrasts with the Milesians’ approach to life. The Pythagoreans believed strongly that numbers, by which they meant the positive integers 1,2,3, ..., had a fundamental, mystical significance. The numbers were a kind of eternal truth, perceived by the soul, and not subject to the uncertainties of perception by the ordinary senses. In fact, they thought that the numbers had a physical existence, and that the universe was somehow constructed from them. In support of this, they pointed out that different musical notes differing by an octave or a fifth, could be produced by pipes (like a flute), whose lengths were in the ratios of whole numbers, 1:2 and 2:3 respectively. Note that this is an experimental verification of an hypothesis. They felt that the motion of the heavenly bodies must somehow be a perfect harmony, giving out a music we could not hear since it had been with us since birth. Interestingly, they did not consider the earth to be at rest at the center of the universe. They thought it was round, and orbited about a central point daily, to account for the motion of the stars. Much was wrong with their picture of the universe, but it was not geocentric, for religious reasons. They felt the earth was not noble enough to be the center of everything, where they supposed there was a central fire. (Actually there is some debate about precisely what their picture was, but there is no doubt they saw the earth as round, and accounted for the stars’ motion by the earth’s rotation.) To return to their preoccupation with numbers, they coined the term “square” number, for 4,9, etc., drawing square patterns of evenly spaced dots to illustrate this idea.